Titanium metal is so far the most successfully osseointegrated implant material in long-term clinical performance [Adell R. et al., 1990]. Despite excellent biocompatibility of thin native oxide films on titanium implants [Kasemo B. et al., 1986 and Johansson C. B. 1991], however, it is generally known that native titanium oxide seldom forms a direct chemical bond to bone tissue (often defined as inert ceramic biomaterial) [Li J. et al., 1998]. Various surface processing technologies such as plasma spraying, simple immersion/soaking method, ion implantation beam associated deposition have been applied to improve biocompatibility of titanium implants. Developments of clinical implants based on alterations of the surface chemical properties have been associated with so called ‘bioactive implant’ coated with bioactive materials such as various species of hydroxyapatite, calcium phosphate compound, bioglass and bioceramic on metal implants [Hench et al., 1990]. However, a number of studies have reported poor long-term biological stability, for instance, a delamination (intra- and interfacial fracture) of such coating materials as well as biodegradation [Albrektsson T., 1998 and Gottlander M., 1994]. Among many surface modification methods, anodic oxidation is a valuable process to enforce the multifactorial biocompatibility such as the oxide thickness, chemical composition, surface morphology, crystallinity, surface roughness change and dielectric constant of the surface oxide in titanium implants [Sul et al. 2000a and 2001b]. The average maximum stress recorded 33 MPA in the mechanical properties of a thick oxide layer [Hala J et al 2000]. Michiaki et al. (1989) and Seishiro (1989) reported the thicker oxide layer in few micrometer to few tens of micrometer with few μm of pore size fabricated in the H2SO4+H3PO4 mixed electrolyte system. The cross-cut structures of the thicker oxide layer measured with Scanning Electron Microscopy (SEM) was characterized to have the network structure of pore channels and connected channel branches.
U.S. Pat. No. 5,478,237 discloses an anodic oxide film containing both calcium and phosphor, which by a further hydrothermal treatment provides a film comprising a calcium phosphate compound such as hydroxyapatite.
WO 98/51231 discloses modified titanium oxide layers of about 10-200 μm and an increased surface oxide crystallinity and roughness.
U.S. Pat. No. 5,354,390 discloses a process of forming an oxide layer using anodic oxidation followed by a heat treatment.
WO 00/72775 discloses coatings including calcium phosphate compounds. According to WO 00/72777 and WO 00/72776, the surface of oxide layer on the titanium includes about 20% titanium, about 55% oxygen and 20% carbon, the rest of the layer is composed of titanium oxide. The used electrolytes are inorganic and organic acid, for instance the H2SO4+H3PO4 mixed electrolyte system. Cross-sections of the titanium oxide layer disclosed in these documents show a very thick layer exhibiting a network of channels, extension of the channels and connected channel branches; However, the purpose of the channels is mainly to allow the administration of a bone-stimulating agent.
There exists a need for a titanium/titanium alloy implant having an oxide layer exhibiting improved surface properties for a faster and stronger osseointegration.